It depends how much of your frontend can be served through the API.
You might have an API that serves specific data requests, but then your application might also have Account, Users, Roles, Permissions, Audits, Subscriptions, Payments etc etc that if you go down the 100% api route then you will need to build out endpoints for all of these when actually your partners only need access to a couple of endpoints.
So, think about the ratio between things that could be regular MVC SSR and things that will need an API
We do have Users, Payments and other tables that dont require API endpoints.
So you suggest using a Tall Stack on our own and keeping API endpoints for partners only?